After the upgrade, none of my previous applications was present except for People and Phone. Interesting, this behavior also applied for the Setting app: it was showing the message "You need to download this app from Store" but I was unable to download the application.
Also, the phone was getting too hot.

So I decided to move back to Windows Phone 8. I tried by pressing the power and volume button and after the vibration, the volume down, power and volume up buttons to start the downgrade process.

Now I am facing serious problem. The phone is showing rolling gears and after some time, when I put the phone in charging, it restarts, but it always shows rolling gears on the display.

Connect your phone and follow the procedures mentioned on the screen. Once, the recovery tool detect your phone, it will just download the firmware and flash your phone with it. So you will be back to Windows PHone 8.1/

Imp: You must not disconnect your phone in the middle of the process. So, make sure to keep your laptop and phone fully charged.
It will take around 1.5 GB of data bandwidth.